1. Field of the Invention
Aspects of the present invention relate to a lithium secondary battery, and more particularly, to a lithium secondary battery having improved safety characteristics.
2. Description of the Related Art
Generally, a secondary battery can be rechargeable, miniaturized, and have large capacity. Recently, secondary batteries have been used as main power supplies of portable electronic devices, such as a camcorder, a portable computer, and a cellular phone. Typically, nickel-hydrogen (Ni-MH) batteries, lithium ion (Li-ion) batteries, and lithium ion polymer batteries have been actively developed.
Lithium, which is commonly used as an active material of a secondary battery, has a low atomic weight, and thus, is suitable for manufacturing a battery having a large electrical capacity per unit mass. Further, lithium intensively reacts with water, and thus, a non-aqueous electrolyte is used in a lithium battery. Lithium batteries are not affected by an electrolysis voltage of water. Therefore, there is an advantage in that lithium batteries can generate an electromotive force of 3 to 4 volts.
The non-aqueous electrolytes used in the lithium ion secondary batteries include a liquid electrolyte and a solid electrolyte. The liquid electrolyte is formed by dissociating lithium salts in an organic solvent. Ethylene carbonate, propylene carbonate, carbonate containing alkyl groups, or similar organic compounds, are commonly used as the organic solvent.
The electrolytes have low ion conductivity. The low ion conductivity of the electrolytes can be supplemented, by increasing an area of an electrode active material, and a facing area between two electrodes. However, there are several limitations related to increasing the facing area between two electrodes. As a result, the low ion conductivity of the electrolyte increases an internal impedance of the battery, resulting in a large internal voltage drop, and limiting output, by restricting a current of the battery when a high current discharge is required.
A separator, interposed between two electrodes, restricts the movement of lithium ions. In the case where the separator does not have sufficient permeability and wettability, the separator restricts the movement of lithium ions between the two electrodes, thereby degrading electrical properties of the battery. Accordingly, important properties of the separator, which relate to the performance of the battery, include heat-resistance, chemical resistance, mechanical strength, void content, and wettability by an electrolyte. The void content is an area of vacant space at a random sectional surface.
The separator of the lithium ion battery also functions as a safety device, which prevents overheating of the battery. A polyolefin-type, micro-porous film, which is commonly used as material of the separator, is softened and partially melted when heated above a predetermined temperature. Accordingly, micro-holes of the micro-porous film, which are passages for lithium ions and a connecting passages for the electrolyte, become closed. As a result, the movement of the lithium ions is stopped, and a current flow of the interior/exterior of the battery is interrupted, and a temperature increase of the battery is stopped.
However, in the case where temperature of the battery is increased, the separator may be damaged, even if the micro-holes of the separator are closed. The separator is partially melted, and two electrodes of the battery directly contact each other at the melted point, thereby allowing producing an internal electrical short. The separator can also shrink, thereby allowing two electrodes to contact each other, and be electrically shorted.
When an over-current flows in the battery, due to a high capacity of the battery, a large amount of heat can be generated. The heat can damage the separator, which can increase the probability of an internal electrical short, to a level that is higher than in the case where the battery temperature causes the micro-holes of the separator to close, because the separator is continuously melted by over-current generated heat.
Accordingly, it is more important to solve the problems of melting or shrinking of the separator, at the time of over-heating of the battery, rather than the current shutdown produced by closing the micro-holes of the separator.
To solve the heat-related problems, a ceramic film is used to prevent internal electrical shorts between electrodes, even at high temperatures. The ceramic film is usually manufactured by forming a film solution, which includes uniformly dispersed ceramic particles, a binder, and a solvent. The film solution can be applied by dipping an electrode plate, coated with an active material, in the film solution. The ceramic film is coated on surfaces where cathode and anode plates face each other, thereby preventing an electrical short between the electrode plates, while allowing lithium ions to pass there through.
In the secondary battery having the coated ceramic film (as an additional separator), it is possible to effectively prevent the internal electrical short, by coating the ceramic film on the electrode active material, as well as on an uncoated part of an electrode, where the electrode active material is not present.
However, there is a problem that the battery, including the electrode plate coated with the ceramic film, is not effectively electrically shorted by an external impact or stimulus, as determined using a nail penetration test, and thus, the safety of the battery is reduced.